Concept explainers
In mice, a dominant allele that causes a short tail is located on chromosome 2. On chromosome 3, a recessive allele causing droopy ears is
To review:
The expected number of 16 phenotypic categories produced after the test cross of F1 mice with jerker mice having droopy ears and long flaky tails.
Introduction:
Test crosses were proposed by Gregor Mendel for the first time.They are used to find the genotype of heterozygous individuals. In a test cross, the heterozygous individual is crossed with its corresponding homozygous recessive individual to determine its genotype.
Explanation of Solution
In mice, the dominant allele is a short tail which is located on chromosome 2. There is a recessive allele located on chromosome 3 which causes droopy ears. It is6mu (map unit) apart than another recessive allele which is for jerker phenotype that is located on chromosome 4. The cross is made between normal mouse and jerker mouse having droopy ears with a short flaky tail. The cross results in normal mouse with flaky tails. These F1 mice were test crossed to jerker having droopy ears and long flaky tail mouse. It results in 400 offspring with 16 possible different phenotypic traits.
The possible number of phenotypic categories can be estimated by finding the percentage of recombination between various genes. Map distance is expressed by the map unit (mu), where 1 mu is equivalent to 1% of the recombination offspring in a testcross. The percentage of recombinants between normal/droopy ears and normal/flaky tail will be
The other two genes for the length of the tail and the gait of the mouse will be assorted independently of the two linked genes for ear and type of tail. There will be 0.5 or
Recombination frequency of offspring is dependent on the phenomena of crossing over. This crossing over in turn depends on the distance between the genes.
Percentage of recombinants of different phenotypic characters are as follows:
On the basis of the above information, the number of offspring in the 16 phenotypic categories can be calculated by using map distance formula as shown below.
Hence, each of the following phenotypes will produce 47 offspring each.
Similarly, by using the map distance formula, a number of offspring of the following traits can be determined:
Hence, each of the following phenotypes will produce 3 offspring each.
So, a test cross between normal F1 mice with the recessive phenotypic traits will result in 400 offspring with 16 different phenotypic categories. The offspring are categorized into 8 phenotypic charactersthat produce 47 offspring and other 8 produce 3 offspring each with a different combination of phenotypes.
Therefore, it can be concluded that the expected number of the 16 phenotypic categories are:
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Chapter 6 Solutions
Genetics: Analysis and Principles
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